1. Field of the Invention
The invention relates to an X-ray apparatus, including a high-voltage generator which is connected across a series combination of an X-ray tube, a resistor and a control element which acts as a variable resistance and which was an anode, a cathode and a control electrode, the control circuit being suitable to generate a control voltage which is dependent on the electric voltage between the anode and the cathode of the X-ray tube and which appears at an output which is connected to the control electrode of the control element, to control the resistance of the control element in a manner to maintain the voltage between the anode and the cathode of the X-ray tube is always substantially equal to a predetermined value, regardless of the current flowing through the X-ray tube.
2. Description of the Related Art
An apparatus of this kind is known, for example from DE-B-21 16 064. The control element of the known apparatus is formed by a triode tube which is connected in the anode lead of the X-ray tube, in series with a parallel connection of the resistor and an inductance. The grid of the triode is connected to the output of the control circuit which comprises a control amplifier whose input is connected to a voltage divider consisting of two resistors which are connected in series between the anode and the cathode of the X-ray tube. The triode acts as a variable resistor whose resistance is controlled by the control amplifier so that the voltage drop across the series connection of the triode and the resistor is always constant, regardless of the magnitude of the current flowing through the X-ray tube. Because the high voltage supplied by the high-voltage generator is also constant, the X-ray tube always receives a substantially constant high voltage. This is important because the intensity of the X-rays produced by the tube depends on the tube voltage and the tube current. Generally speaking, the resistor is connected to a measuring circuit for determining the tube current, so that it cannot be omitted. However, when the tube current is increased in order to increase the intensity of the X-rays, the voltage drop across the resistor increases so that the tube voltage decreases. Consequently, a decelerating field is created for the electrons emanating from the cathode, which field counteracts the emission of electrons by the cathode. In order to achieve the desired emission current nevertheless, it is necessary to increase the cathode temperature so that enough electrons are released, despite the decelerating field. In order to reach this higher cathode temperature, the current in the filament of the cathode must be increased. Increasing the cathode temperature has a negative effect on the service life of the filament and hence on the service life of the X-ray tube. Therefore, this solution is not very desirable. The control element makes it possible for the tube voltage to remain always substantially constant. When the tube voltage decreases, the signal applied to the control amplifier via the voltage divider changes. As a result, the control amplifier influences the control electrode of the control element so that the resistance of this element also decreases. The overall resistance of the series connection of the control element and the resistor then also decreases, so that the voltage drop across this series connection remains constant despite an increased tube current. It is a drawback of the known solution, however, that it requires the use of a comparatively complex, expensive and slow control amplifier.